The Nanohertz Gravitational-Wave Detection Explained

https://physics.aps.org/articles/v16/116

by raattgift • 3 years ago

178 68 3 years ago

physics.aps.org

mariushop• 3 years ago

outstanding delivery

zadwang• 3 years ago

Excellent illustration. After reading it I thought this looks like the PHD style. And I checked the author, who IS Jorge Cham. About 22 years ago I was reading his Piled higher and deeper, PHD, series and bought several books of his. It is a great feeling to see that he is still doing comics. Thanks Jorge!

leetrout• 3 years ago

I had never heard of "cosmic strings".

https://en.wikipedia.org/wiki/Cosmic_string

chrisweekly• 3 years ago

Fantastic explainer. The illustrations are both charming and helpful.

It's reminiscent of other excellent multi-modal "explainers", e.g. Julia Evans (https://jvns.ca), https://betterexplained.com, Randall Munroe (https://xkcd.com), http://inception-explained.com ...

And on this topic of mixing text with imagery, I highly recommend Nick Sousanis's amazing "Unflattening" (https://archive.org/details/unflattening0000sous)

dilaver• 3 years ago

"First, [gravitational waves] have really long wavelengths."

Why?

speff• 3 years ago

λ = v / f

The velocity (v) is the speed of light for gravitational waves, which is already a really big number. If the frequency (f) is based on the period of the black holes which circle around each other, then I assume one rotation happens over a long period of time. Long period -> low frequency -> small denominator which makes the wavelength (λ) even longer.

geon• 3 years ago

Some rotations are fast. LIGO detects gravitational waves at 200-10000 Hz.

That’s kind of fast. Just imagine a couple of black holes orbiting each other that fast.

defrost• 3 years ago

Not a bad question at all really.

    In principle, gravitational waves could exist at any frequency. 

    The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave.

    For example, the animations shown here oscillate roughly once every two seconds. This would correspond to a frequency of 0.5 Hz, and a wavelength of about 600 000 km, or 47 times the diameter of the Earth.

https://en.wikipedia.org/wiki/Gravitational_wave

To get a short wavelength requires a high frequency, to get an observable gravity wave requires a very large mass.

We haven't yet seen a Super Massive Black Hole orbiting with (say) an Mercury orbit radius at a thousand times a second.

There is another quote from that wikipedia article:

    Stephen Hawking and Werner Israel list different frequency bands for gravitational waves that could plausibly be detected, ranging from 10^−7 Hz (very slow) up to 10^11 Hz (very very fast).

The faster the wave the shorter the wavelength and the greater the difficulty in detection (as the amplitude likely lessens and with distance falls below our current direct means).

You'd have to chase the Hawking-Israel paper for their thoughts on short wavelength high frequency gravitational wave sources and how they believe they might plausibly be detected .. I anticipate some devil in the detail.

http://library.lol/main/F92F35CD83F13A6021FC2385BBA171B0

( perhaps wikipedia misquoted that high frequency )

tzs• 3 years ago

> The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave [...]
To get a short wavelength requires a high frequency

For light that is assuming a vacuum. The more general equation is c' = λf where c' is the speed of light in the medium the light is traveling through. c' <= c. Hence, for light, you can get a shorter wavelength without having to raise the frequency if you work with the light in a medium with a lower c'.

Is there anything similar with gravitational waves?

bloopernova• 3 years ago

What an utterly delightful and eminently understandable presentation of a complex subject!

I know that it would take decades or centuries to gather enough data to make discoveries. But I wonder if one day a scientist will be looking at these results and see something that makes them think "that's odd..." And suddenly our understanding of the universe is turned upside down yet again!

p1mrx• 3 years ago

Australian scientists vindicated as new pulsar data proves South is up.

layer8• 3 years ago

It would be nice if there was a plain-text version for accessibility.

NickC25• 3 years ago

I don't have a deep understanding of physics and would have no chance at understanding the details of this if it was in a traditional academic paper, and not presented so well. This is absolutely fantastic!

jack-bodine• 3 years ago

If gravitational waves can be detected by looking at pulsars, then what is the purpose of LIGO and ground-based gravitational wave observatories? Is there any difference in the waves detected by LIGO and those from observing pulars?

venusenvy47• 3 years ago

Those two methods observe waves of much different wavelength/frequency, and the mechanisms that create such different wavelengths are assumed to be different. So cosmologists are studying different things by looking at different wavelengths.

magicalhippo• 3 years ago

As mentioned in the sibling comment, different frequencies means probing different things.

I found this talk[1] rather nice to explain the NANOGrav experiment, and at 6:45 there's a very nice plot that shows where NANOGrav fits in compared to the other gravitational wave experiments and which type of sources the various experiments can probe.

[1] https://pirsa.org/20100068

_Microft• 3 years ago

LIGO detects gravitational waves in the frequency range of hundreds of Hertz (10^2 Hz), which are produced in the last moments of the in-spiraling of merging neutron stars or blackholes.

Nanohertz are 10^-9 Hz which relate to a rotational period of decades of years.

https://en.wikipedia.org/wiki/Gravitational-wave_astronomy

Cerium• 3 years ago

For one we didn't have a way to measure the speed of gravitational waves before [1].

[1] https://physics.stackexchange.com/questions/622729/did-ligo-...

brookst2• 3 years ago

Really great explainer!

Naive question: how do we know it is gravity specifically distorting space? Could there be other forces or phenomena not related to mass?

teraflop• 3 years ago

The theory of general relativity says that "gravity" and "distorted space(-time)" are one and the same phenomenon. So it's a question of whether you believe that theory is correct.

We've known for a long time that GR predicts that when black holes and/or neutron stars collide, they should emit gravitational waves with a very specific shape. The LIGO and Virgo observatories detected faint signals that seem to match this shape, which gives us confidence that the theory is a correct explanation of the data.

This adds to the mountains of other evidence for GR, and complements it by showing that GR still holds in environments where space-time curvature is extremely strong.

Technically, we don't know for sure that it's impossible for GWs to be produced by other sources than moving masses, but we have neither theory nor evidence to suggest such a thing is possible.

brookst2• 3 years ago

Thank you for the wonderful explanation!

florbo• 3 years ago

So it's a question of whether you believe that theory is correct.

You shouldn't believe in a theory, they simply explain what we think we know, which always changes!

aaron695• 3 years ago

Very well done

So pulsars are not regular at the tick level as they can spin faster and slower but have 100 nanoseconds accuracy (Way worse than an atomic clock) when doing an average profile -

https://astronomy.swin.edu.au/cosmos/p/Pulsar+Timing

This is a cool line -

"In effect this allows changes in the relative distance between the pulsar and Earth to be computed to an accuracy of 30 metres (~100 feet)"

beders7• 3 years ago

I still don't understand what the carrier of gravitational waves is.

How are the waves propagated? Space is not an elastic fabric or water where water molecules transfer potential energy.

Aren't gravitational waves a hint that spacetime is not fundamental, but an emergent property of something else?

dotnet003• 3 years ago

Space is like a fabric, it's a literal compression and stretching of space as the wave propagates through it. The distances physically get shorter and longer as the wave passes by.

It doesn't necessarily mean that spacetime cannot be fundamental, as there's no rule that says that space itself must emerge from some sort of underlying medium (eg the way fabric is made of threads).

beders7• 3 years ago

What is stretching/compressing it?

Spacetime is warped by mass but no mass is transmitted. So what exactly is being carried along?

turndown2• 3 years ago

Space is not an elastic fabric

Space is an elastic fabric, if Einstein's explanation of a spacetime dimension is correct. This would mean that gravity is the warping and stretching of this dimension.

If it is not then the other solution is in quantum field theory, which would state that gravity is a field with a messenger particle, the graviton, in which case we are just detecting a group of gravitons as they pass by.

beders7• 3 years ago

The confusing thing here is that space time warps in the presence of mass.

But no mass is transmitted in a gravity wave.

So what is the fabric made of?

consumer451• 3 years ago

Along similar lines, I wonder if gravitational waves can be focused.

pfdietz2• 3 years ago

They should be focusable by objects with mass, just as light is. A difference is that gravitational waves could be focused by the Sun's core, and so would come to a focus closer to the Sun than light would be, which would have to be focused by the entire Sun (as any light passing through the Sun itself is blocked.)

raattgift5• 3 years ago

Yes, foreground massive objects can magnify gravitational waves emitted by a background source (the foreground objects can also induce a beating pattern on the background waves, or split the background waves into multiple wavefronts). The "foreground" emitter in a massive cluster of thousands of galaxies can be deep inside the cluster, and those are probably the gravitationally-lensed gravitational-wave sources we will identify first [more below at [1]].

A good starting point for a non-specialist is probably <https://astrobites.org/2021/05/25/gw-lensing/>.

(You could compare a specialist paper <https://www.aanda.org/articles/aa/full_html/2020/11/aa38730-...> which describes the lensingGW software package https://gpagano.gitlab.io/lensinggw/ ).

- -- [1] When a candidate gravitational wave (GW) detection is found at LIGO et al., an electromagnetic (EM) detection may follow. Neutron stars collide explosively, generating a huge burst of very high-frequency (gamma ray) EM. Black holes don't explode but their accretion structures can interact very strongly producing high-frequency (X-ray) EM. These bright high-frequency EM flashes decay into a glow that can last for many days after a merger. Consequently they are looked for, as part of the overall project of multi-messenger astronomy (where EM and GW are two "messengers"; neutrinos and so forth make it "multi-").

Close to the source, peak EM tends to lag a bit behind the merger, mostly because of how the EM is produced at some distance outside merged black holes. Additionally, EM scatters/refracts/etc with dust and gas that one finds deep inside galaxies, which are more transparent to GW than to EM. Consequently in general GWs will be detected before EMs from the same source. Candidate GW detections provoke searches across the electromagnetic spectrum, since EM may arrive hours or even days later.

GW from an inspiral and merger in general have much much longer wavelengths than the X-rays from the gas around the merging objects. Black holes merge roughly when their horizons touch, and horizon size is proportional to mass. GW wavelength is proportional to the orbital diameter, which must be larger than the sum of horizon radiuses, so supermassive black hole binary (SMBHB) mergers have much longer-wavelength GW than stellar-mass black hole binary mergers.

The relatively short wavelengths of electromagnetic radiation will follow the laws of geometrical optics and in particular will experience Shapiro delay. GW wavelengths, being much longer, will not. This explains a further delay on EM from SMBHB mergers that are deep within a massive galaxy or cluster of galaxies.

Foreground lenses (e.g. massive galaxies that the source does not live within) will further increase the delay suffered by EM produced by the merger that sourced the GW. Moreover, since foreground lenses can split the wavefronts of the GW and EM, there may be ~four detections, at different times, of the same merger in GW and EM. Alternatively, the foreground lens can magnify the source merger, but with the delays described above and geometric optics vs not, the focal point for GW and for EM will differ.

All of this is active research, close to cutting edge, so sadly is mostly to be found via a literature search on Einstein lensing of gravitational waves. AFAICT most "pop sci" attempts to summarize some of the academic literature is simply awful. Hopefully I'm merely just not great above. :-/

eis• 3 years ago

I don't think they are a hint that spacetime is not fundamental. But I do think spacetime has to be some kind of real physical reality.

The modifications of spacetime that we see as effects of gravity are relative changes to our immediate surroundings or reference frame.

Similarly how you can't tell who is actually stationary and who is moving when two objects are in freefall and all you can note is the relative speed between the two, it would be equally valid to say the objects inside spacetime are getting distorted relative to spacetime.

pfdietz2• 3 years ago

Gravitational waves follow because space time and gravity are described by equations that admit waves as solutions. Thinking about what space time is "made of" is contrary to what mathematical physics is about. This was a realization with electromagnetism: early attempts tried to interpret the EM field as something mechanical, but it was eventually realized that math making predictions was what one should focus on, and not try to create some sort of more comfortable interpretation in terms of materials or gears or something.

beders7• 3 years ago

Not looking for a comfortable interpretation.

The confusing thing here is that spacetime warps in the presence of mass.

But no mass is transmitted in a gravity wave.

So how do the waves travel?

jrmg• 3 years ago

I think you’re looking for a ‘ether’? Many people have asked similar questions about all sorts of physical phenomena over the years.

https://en.wikipedia.org/wiki/Aether_theories

nwallin• 3 years ago

So forget about waves for a second. Forget about spacetime. Forget about general relativity.

The force of gravity is correlated to the mass of the object divided by the square of the distance to the object. The higher the mass, the lower the curvature. The larger the distance, the lower the curvature.

The force of gravity doesn't change instantaneously. The speed is the speed of light. If you move the Sun around using Sufficiently Advanced Technology, (SAT) the orbit of the Earth won't change until 8 minutes later, when the gravity has had time to propagate from the Sun to the Earth.

So what does it feel like if you're on the Earth and somebody is using SAT to wiggle the Sun around? The force felt from the Sun will go up and down. The math that you would use to quantify this changing force is the same math you'd use to characterize sound waves, or water waves, or electromagnetic waves. So we call this changing tidal acceleration 'gravitational waves'.

That's how gravitational waves were described in the 19th and early 20th century, before general relativity. Not long after Einstein published his paper on General Relativity, he adapted the same process via which gravitational waves propagate in Newtonian F = m1 m2 G / r^2 gravity to General Relativity/curvature of spacetime gravity. The math is nasty, and it wasn't really ironed out until decades later, but the theoretical predictions of what gravitational waves will look like in any given detector was worked out well before we actually went and detected them.

Space is not an elastic fabric

It's a 4D/(3,1) Lorentzian manifold. If you want a better analogy than the stretched elastic fabric analogy, unfortunately the next step is to grab a textbook and slog through the math.

beders7• 3 years ago

Thanks a lot for all the answers! Many of them are outside of my math skills but I appreciate the different ways this was explained. Thanks!

eande• 3 years ago

Is there a similar illustration to the Quantum Entanglement phenomena? Something I yet to have to fully understand how that is possible?

turndown2• 3 years ago

Say your car and another car got in a crash, but you both panicked and drove away(that is, you did not inspect the damage.)

You get home and finally summon the courage to look at your car. It's busted. With that information you can infer that the other car is busted too, and you don't need to ever come in contact again with the other car to know this fact only that at some point in the past your car and theirs interacted.

swid• 3 years ago

This explanation infers a hidden variable, which is not how quantum entanglement works.

IE - it sounds like the damage occurs at the time of impact, and you just decided to look at a later point in time. But that isn’t what happens with quantum entangled particles. The particles will have opposite spin when the entangled state collapses, but measurement will affect the angle of the spin in a way that proves spin is not preselected when the particles were initially entangled and local to one another.

In your story, looking at your own car would have an observable effect on the entangled car, even though it is far away. But you also cannot even tell it was entangled with the other car without comparing the two over more traditional channels!

Edit: Since a graphic was requested, I found this image hosted by JPL, which shows a related phenomenon (quantum teleportation) to what I described, and hopefully dispels the car crash explanation which doesn’t imply “spooky action at a distance”.

https://www.jpl.nasa.gov/news/researchers-advance-quantum-te...

whoknowsidont2• 3 years ago

I've always wanted to ask this really dumb question, so I'll just ask it here (laugh or answer, your choice):

Would there be gravitational waves so huge that we'd have trouble detecting them? As in, if there was a huge, massive gravitational wave and we were in the process of "riding" it how would we ever know or detect such a thing?

Would this possibly (not plausibly) be an answer to some of the weirdness and contradictions we have observed in astronomy over the past few decades?

krastanov• 3 years ago

You can see "sensitivity" plots for the various methods we have for observing gravitational waves. The horizontal axis is usually in terms of the frequency of the wave and the vertical axis is how sensitive we are to them. This article is about "nanohertz" waves, which means their wavelength is roughly 10⁹sec × speed of light, i.e. 30ish light years. So, it seems we can detect really "big" gravitational waves. If you look through the aforementioned plots you will see that sensitivity drops off significantly for "more astronomical" wavelengths, so there are definitely scales at which we can not detect, as you suggested.

There might be some interesting ways to introduce very long gravitational waves as solution of current discrepancies in our understanding of cosmology, but they are also probably introducing more discrepancies than they solve. I would guess people have considered these ideas, because "cosmological scale general relativity" is pretty heavily researched and mainstream.

raattgift5• 3 years ago

so huge that we'd have trouble detecting them

Gravitational waves can be arbitrarily long because patterns in the cosmic microwave background suggest the early universe had gravitational radiation with wavelengths of arbitrary size, including quite long waves. The metric expansion of space has stretched those "primordial gravitational waves" (PGW), so even short-wavelength-in-early-universe PGW can have periods of many years at about the present size of the universe.

Additionally, most models of cosmic inflation stretch PGW, with some of the stretched wavelengths being made longer than the distance to the cosmic horizon (Initially, before inflation, PGWs can have practically unlimited length or can be almost arbitrarily short, with the shortest winding up not so short once inflation has ended), so at modern times the super-horizon-length primordial waves would have periods of many billions of years.

Any masses in a ~binary orbit (stars to galaxies, down to grains of dust and up to collections of galaxies like in the local group, <https://en.wikipedia.org/wiki/Local_Group>, the "dumbbell" shape is relevant) generates gravitational waves with a frequency comparable to the orbital period. In our sky there are galaxies that appear to be in mutual orbits with orbital periods of many millions of years.

The pulsar timing arrays in the cartoons linked at the top of the page look for gravitational waves with periods of months to years. It takes several orbital periods to be statistically comfortable that a gravitational wave with that period (nanohertz frequency) has been detected. Those are probably generated by supermassive black holes in hard mutual orbits. If we soften such an orbit, increasing the semimajor axis (or radius for a circular orbit), the orbital period grows, so we have to watch the array of pulsars longer (and be more wary of so-called red noise and other long-term signal contamination).

some of the weirdness and contradictions we have observed in astronomy

Unlikely. Do you have any particular weirdness and/or contradiction in mind?

whoknowsidont2• 3 years ago

Thank you for this incredibly informative and detailed answer! It's going to take a few passes (and following some links) to put everything together.

Do you have any particular weirdness and/or contradiction in mind?

Not in particular or specifically. I was just imagining that if there were such a thing possible, it surely would create some weird anomalies in our measurements.

The idea that something could warp our spacetime so dramatically that we would perceive it as being the normal state of things, even though the "warp" is ultimately temporary, is for some reason incredibly interesting to me.

The Nanohertz Gravitational-Wave Detection Explained

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